[TowerTalk] Paintg Traps-Coax issues-Respon fr DAVIS RF Co.

Steve Davis -Davis RF Co. sdavis at davisrf.com
Sun Jul 20 14:59:26 EDT 2008

    This also reflects on Jack Brindle's astute comment

Hi John,    The min bend radius of Bury-Flex Tm is 2 inches.  I have sent 
you a copy of the full spec via separate email.   Just to avoid any possible 
confusion by readers coming in at this point, the trap with  coax winding 
question had to do with RG-213. .  Well made RG-213's also have a 2 " bend 
radius; however,
an often missed point, albeit not often a consideration, is the "repeat bend 
radius" spec.  In the case of this trap winding, now that I was reminded by 
Jack Brindle that this is a 4" OD Sched 40, the repeat aspect of  this trap 
winding could be an issue.  I think the safest way to approach this is to 
contact the mfr. or distributor of the particular RG-213 involved (not 
mentioned in this matter) and find out the repeat bend radius, and at what 
number of
repeats does the issue start an adverse indication.  In my first response I 
somewhat qualified my remarks based on not having seen the actual 
configuration of this trap, so I really can't opine much further than 
already, thus leaving some of this subject to further specifics of the 
  If the spec'd repeat wind radius is marginal, wind the trap and test it to 
be sure.  My experience  tells me though that characteristics of an RG-213 
build will be somewhere in the 3-4 times the min. bend radius to come up 
with the prox. repeat bend radius, thus that would be 6-8" min repeat bend 
radius, unacceptable for a 4" OD winding core.
   Also Jack's comment about internal heat:  yes, ties to temp. rise of coax 
line: related to freqcy, power, ambient temp  and attenuation.
  Bottom line, objectively it is hard to quickly surmise; however, I think
that at this point of the various opinions, 4" PVC is not a good route, 
unless the source of this design (?) has done it with properly tested 
results.  That maybe is the next place to look before condemning this, but I 
will have to leave it at that.  And, given that lightning is now approaching 
here.  HI      Cheers, Steve

----- Original Message ----- 
From: "wa5zup" <wa5zup at msn.com>
To: "'Steve Davis -Davis RF Co.'" <sdavis at davisrf.com>
Cc: <towertalk at contesting.com>
Sent: Sunday, July 20, 2008 2:00 PM
Subject: RE: [TowerTalk] Paintg Traps-Coax issues-Respon fr DAVIS RF Co.

> "I do not expect the heating
> factor to be involved if the proper cable materials are utilized, 
> inherent
> in a quality cable, and if the rated min. bend radius is adhered to."
> What is the bending radius spec for BURY-FLEX cable?
> I looked on the web site, but did not find it, maybe I missed it.
> Thanks
> John
> Wa5zup
> -----Original Message-----
> From: towertalk-bounces at contesting.com
> [mailto:towertalk-bounces at contesting.com] On Behalf Of Steve Davis -Davis 
> RF
> Co.
> Sent: Sunday, July 20, 2008 9:12 AM
> To: Jack Brindle
> Cc: towertalk at contesting.com
> Subject: Re: [TowerTalk] Paintg Traps-Coax issues-Respon fr DAVIS RF Co.
> Hi Jack,  Your discussion is not over technical as your points are well 
> put
> and fundamentally germane to the basics of coaxial design and function. 
> The
> only catch is that it appears that you are skeptical that all cable jacket
> PVC's, and their PE dielectrics (both ref. RG-213 and many other similars)
> will not hold up to heating by the sun and resulting ambient  transferred
> through the shield to the dielectric.  There are many compounds of PVC
> jacket and PE dielectric used in cable extrusion.  Choosing the proper 
> types
> as I noted (or relying on the mfr.; distributor; or design engr),   will
> hold up very well in any sunlight scenario for many years.  PVC compounds
> certified at 60 deg. C., and UV resistant,  are my requirement for outdoor
> applications of our RG-213 and similar cable.
> Your concerns are not uncommon and have been  noticed in some of our
> commercial inquiries where they have spec'd Teflon outer jackets when the
> ambient would never reach in excess of 60  C, thus we've been able to save
> them a lot of money by using the correct grade of PVC.
> I agree completely with you ref. the electrical degradation due to  center
> cond. concentricity misalignment, IF there is enough ambient heat combined
> with spiral exertion to cause this...however I do not expect the heating
> factor to be involved if the proper cable materials are utilized, 
> inherent
> in a quality cable, and if the rated min. bend radius is adhered to.
> 73,   Steve ,  K1PEK,
> Wire, Cable, RF Connectors and wire aerial parts.   LMR, Heliax, Eupen
> Commercial / Military / & Custom Cable Design
> tel:  978-369-1738     Fax:  978-369-3484
> www.davisRF.com

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